The first generation of thermal insulation included materials with naturally low densities such as cotton, wool, cork and asbestos. Since entrapped air (with thermal conductivity as low as 23 mW/M·° K) was the primary insulator, a lower density (more air) corresponded to a higher insulation value (i.e. lower thermal conductivity). The second-generation insulations were industrially processed, porous substrates containing even more air (lower density and more insulating). These insulations included fiberglass, rubber and plastic foams, and other man-made porous substrates. The scheme used by the second generation, lowering the thermal conductivity (K value) by reducing density, finally reached its bound (˜30 mW/M·° K)1 when such practices began compromising insulation strength and performances, attributed to too much air as well as excessive radiation heat loss. The third generation has evolved with the emergence of nanotechnology. Modeling results demonstrated that if the pore size of insulation could be reduced to below mean free path2 of air, i.e. <100 nm, the insulation value can be substantially increased by reducing the low thermal conductivity of entrapped air even lower. 1 Some insulation materials on market have thermal K lower than this value due to the addition of heavier gases such as HCFC and CFC. They are not considered as a new generation of insulation because their thermal K will rise to the 2nd generation limit value after the heavier gases diffusing out of the substrate over certain use time.2 Mean free path: the average distance traveled by a molecule between two consecutive collisions.
Between 1992 and 1995, we demonstrated this feasibility while working at Armstrong World Industries on a $4.5 million project awarded by the Advanced Technology Program3. By reducing the pore size to nanometer scales, we successfully made several nanopore composites with super insulation properties. The following table listed data of previous samples in comparison with a second generation insulation, fiberglass:
Insulation MaterialThermal K (mW/M · °K)R/inch*Fiber Glass453.2Granular Silica Aerogel236.3Aerogel with Layered Silicates Panels169Inverse-Emulsion Composite Panels207.2Aerogel-Polymer Microcomposites178.7Inorganic-Organic Composite1310.5Aerogel*R-per-inch is a commonly used measure of insulation value. It is equivalent to the reciprocal of the thermal conductivity in unit of Btu · in/hr · ft2 · ° F. The energy code is given by R-value, i.e. R-per-inch times thickness.3 “Thermal Insulation Materials-Morphology Control and Process for the Next Generation of Performance,” ATP award to Armstrong World Industries, Inc. (1992).
These new materials failed to reach markets because of their high processing costs. This class of material is a super insulation due to a combination of high porosity and nanometer-size pores. These special structural attributes are also causing its processing difficulties. The high porosity material is mechanically weak. When drying under ambient conditions, capillary stress from the liquid meniscus in the pore shrinks the material and results in significant structural damages. For pores of nanometer size, this stress can be in the range of a hundred bars (˜1500 pounds per square inch); the smaller the pore, the higher the stress. The shrinkage due to high stress reduced the porosity and the number of nanometer pores in the material structure; resulting in substantial loss of its super insulation value after ambient processing.
One solution to this problem was to dry the wet gel under supercritical conditions of a fluid (most conveniently, by, using supercritical CO2 fluid). This had allowed the liquid system to bypass the coexisting (infinitely compressible) region and avoid generating any meniscus within the pores. This processing requirement, drying the material under a supercritical condition, instead of ambient condition, was the reason for the high capital and processing costs associated with the production of nanopore insulation. The following table provides direct comparisons of making insulation by solution process to that by gas foaming process, which is the most widely process used for making the second generation of insulation.
ProcessGelation and DryingDirect Gas FoamingCharacteristics(Aerogel, Hydrogel)(Gas Blown Polymer Foams)a. Porosity is Created by:Liquid solventGas Bubblesb. Fluid Weight % Needed2000%5~10%for Creating 95% Porosityc. Processing SpeedFluid diffusion, depending onGas blown, instantaneoussample thickness (δ), ∝δ2, slowd. Pore Size Control10 nm, needs special100-500μ, very difficult toprocessing care to preservecontrolthe nanoporese. Thermal Conductivity20 mW/M°K30-35 mW/M°K
If we can replace the solution drying process and make nanopore insulation by foaming, the cost reductions will be well beyond those needed to make the technology commercially viable. Recently, we had successfully developed the technology of producing low-density (density ˜0.03 g/cc) Styrofoam insulation by foaming with 100% CO2. By integrating the two technologies together, we could design a system that utilizes supercritical CO2 to first create and preserve nanometer gas embryos (by a nucleation process), and, then to expand gas bubbles (by a foaming process) for making low-density insulation. Such a system could produce high-porosity foams with extremely small pore sizes. The challenge, of course, remained as how to effectively control the bubbles' size during the rapid foaming process.
A foaming process consisted of rapid generation of numerous gas bubble nuclei, followed by their fast growth during the foam's expansion. We could envision two approaches to control the pore size during such a rapid bubbling process. First, we plan to induce the homogeneous nucleation process (already demonstrated by pressure vessel experiments) in a foam extrusion process to generate extremely small gas embryos, followed by controlling the bubbles' growth. Or, we can use a reactive system, such as the polymerization of styrene or urethane, which secrete out volatile solvent, or co-solvent, during its polymerization and depressurization, to create a spinodal decomposition, followed by controlled expansion of the entrapped volatile fluid phase. Both processes required a low initial interfacial tension, as well as a controlling mechanism to slow down the bubble growths. Either process, if successful in generating fine pores and high porosity, would lead to a breakthrough in producing nanopore insulation because of the tremendous cost savings attributed to the rapid depressurization of supercritical CO2 (or similar volatile fluids). Obviously, such a breakthrough process would be difficult, because it would require orders of magnitude improvement in pore size controls (from ˜100 microns to 0.1 micron) comparing to prior foaming arts.